python实现批量修改文件扩展名

import os
def swapextensions(dir, before, after):
#0<=x<1
if before[:1] != '.':
before = '.'+before
thelen = -len(before)
if after[:1] != '.':
after = '.'+after
for path, subdirs, files in os.walk(dir):
for oldfile in files:
if oldfile[thelen:] == before:
oldfile = os.path.join(path, oldfile)
newfile = oldfile[:thelen]+after
os.rename(oldfile, newfile)

if __name__ == '__main__':
import sys
if len(sys.argv) != 4:
print "Usage: swapext rootdir before after"
sys.exit(100)
swapextensions(sys.argv[1], sys.argv[2], sys.argv[3])